Matthew Poole and continuators on Exodus 9:19

19 Send therefore now, and gather thy cattel , and all that thou hast in the field: for upon every man and beast which shall be found in the field, and shall not be brought home, the hail shall come down upon them, and they shall die.

This forewarning God gives, partly to mitigate the severity of the judgment; partly that a considerable number of Horses might be reserved for Pharaohs expedition chap. 14. partly to shew the Justice of God in punishing so wicked and obstinare people, as would take no warning neither from Gods words, nor from his former works; and partly to make a difference between the penitent and the incorrigible Egyptians.